The present invention generally relates to the field of semiconductor production, and more particularly to wafer processing equipment and a method for processing wafers.
The processing of semiconductor wafers is well known.
This processing often begins by the patterning of a particular sequence of successive layers upon the wafer. Patterning of layers, often called photolithography, involves many steps. The first step might be to create a silicon dioxide insulating layer on the surface of the silicon wafer. Thereafter, removal of selective portions or sections of the insulating layer may be achieved to expose underlying silicon. Selective removal of silicon dioxide is achieved by spinning a photoresist material across the silicon dioxide surface. A mask is used to allow a source of ionizing or non-ionizing radiation to impinge at selective locations across the wafer. Depending upon whether the photoresist is positive-type or negative-type photoresist, certain areas of photoresist may dissolve when exposed to solvents, thereby exposing selective areas of silicon dioxide. Thereafter, the exposed portions of silicon dioxide can be removed by etching techniques commonly known in the art.
In many cases the timing between different process steps is critical. One example for such a critical timing between process steps is the time between the coating of a wafer and the exposure, especially if the DUV lithography is used. Another example for a critical timing between different process steps is the time between the exposure of a wafer and a following post exposure heating process. To achieve a proper timing is especially difficult since different processing tools in many cases comprised different throughputs. For example a coating device may be capable of processing about 42 wafers per hour, whereas a stepper device performing the exposure may be capable of processing 8 to 33 wafers per hour. In such cases a track to which the coating device and the stepper is assigned is usually configured to ensure that the stepper never waits. To ensure that the stepper never waits a track may comprise a buffer for intermediate storage of wafers to be processed by the stepper and the exposure device, respectively. However, if wafers are stored in such a buffer for a longer period the above mentioned timing problem becomes relevant.